Powder coating compositions have many advantages over conventional solvent-based coating compositions. For example, they are non-polluting and easy to recover, and their production lines are easy to control. Because of these advantages, powder coating compositions have been drawing attention particularly in the field of automotive coating. However, when a conventional powder coating composition is applied to automotive exterior panels or like parts that are required to be coated with a coating film with good finishing appearance, the composition needs to be applied to an increased thickness to achieve a sufficient smoothness. For example, in order to attain a film smoothness equivalent to that of an about 40 .mu.m-thick coating film formed from a solvent-based coating composition, the conventional powder coating composition needs to be applied to a thickness of about 60 .mu.m or more. Therefore, powder coating compositions are demanded which are capable of forming a thin coating film having an improved smoothness.
Attempts have been made to improve the film smoothness of powder coating compositions by reducing the particle size of the compositions as much as possible. However, powder coating compositions having a reduced particle size are disadvantageous in that they are low in coating efficiency in electrostatic powder coating, necessitate a troublesome procedure for recovery owing to the lowered coating efficiency, and give coating films with poor finishing appearance because of the increased number of recovery steps which cause variation in the particle size of the powder coating compositions or contamination with foreign matter such as dust. On the other hand, powder coating compositions having an increased particle size result in coating films with poor smoothness, although being improved in coating efficiency.
No powder compositions have been so far proposed which realize both a high coating efficiency and good film smoothness.